In a traditional analog radio frequency (RF) transmitter, configured to receive a digital signal, the digital signal is converted into an analog signal in a digital-to-analog converter (DAC) before any other signal processing. The analog signal is then filtered, up-converted and amplified in a linear power amplifier. In the linear power amplifier, the small analog/RF linear signal from the digital-to-analog converter DAC is power enlarged to reach a required output power level. The amplified signal is then filtered to remove the bandwidth expansion caused by non-linearity in the power amplification. Finally, the amplified analog/RF signal is output to an antenna. In such a traditional analog/RF transmitter the digital contents of the signal no longer exist after the conversion in the DAC.
In recent years, digital transmitters (DTX) and digital power amplifiers (DPA) have undergone an extensive development with support from Complementary Metal Oxide Semiconductor (CMOS) technology. Due to CMOS process scaling, digital components can nowadays switch at a high frequency which even surpasses radio frequencies while still keeping the operating power low.
This trend provides the motivation to realize a DTX/DPA in a pure digital style. In a DTX/DPA architecture according to conventional technology the demand of using digital signal processing as much as possible removes the use of a DAC. The DAC is replaced by a digital up-sampling module to align the data flow bit rate with a digital carrier signal (DF1o) later in the DTX/DPA. For the same reason the analog channel bandwidth filter, is also removed. To compensate for the digitized signal quantization noise problem, a noise shaping algorithm/module is mostly used to enhance the signal-to-noise performance and during this stage, different DPA modulation algorithms and different types of DPAs emerged. For example, some may use ADC sampling algorithm and others may use sigma-delta modulation (SDM) algorithm or pulse width modulation (PWM) algorithm, so these algorithms categorize the DTX/DPA into RF-DAC/RF-SDM/RF-PWM type DTX/DPA.
A DTX/DPA is a kind of transmitter architecture which implement, mostly digital switching blocks/modules for signal processing/modulation and switching PA as output stage to amplify output RF power. A DTX/DPA is different from traditional analog/RF transmitter because the internal signal flow is mostly on/off switching digital characteristics instead of continuous analog/RF signal.
After the noise shaping processing, the digital signal with multiple level representation needs to be further processed and mapped into a fully switching on/off (‘0’s or ‘1’s) signal, here the digital demodulation module will be used. In this stage, the digital signal will finally be synchronized into bit rates for the digital carrier signal and is turned into a fully ones/zeros bit sequence. The demodulation method can match the previous modulation algorithm but it is also possible to use a combination of modulation techniques. As an example, SDM modulation may use ADC or PWM style demodulation methods.
With digitized high speed baseband data according to conventional technology, digital up-conversion and mixing may also be realized in digital style. For example, when ‘1010 . . . 10’ represents 0 degree phase carrier signal, its complementary signal ‘0101 . . . 01’ represents 180 degree negative phase signal. And with more bits combination, I channel and Q channel carrier frequency signal can be represented in digital bits. This greatly facilitates the RE digital up-conversion process as a simple ‘AND’ logic operation is sufficient. Since the digital RF I/Q carrier signal has a fixed pattern for every baseband modulation cycle, a batch process can help to reduce the processing clock frequency and parallel data bits are generated during this process.
In DTX/DPA architectures, according to conventional technology, a digital signal may be modulated into an in-phase signal and a quadrature signal. According to conventional technology SDM is used for noise shaping processing and PWM translation is used as digital demodulation for the in-phase signal and the quadrature signal, respectively. Connected to the digital demodulation modules are repeaters configured to data rate match the signal from PWM to the RF carrier signal. Connected to the repeaters is an interleaver module which realizes digital up-conversion and mixing. The mixed digital signal is then fed to a power amplifier (PA). The PA is connected to a load which is configured to radiate RF signal into the surrounding air. A problem with the DTX/DPA architectures according to conventional technology is that the power/hardware cost for the SDM module is high if the SDM module is to operate at a high processing speed. If the SDM operates at a low processing speed the out-of-band noise becomes high. A low processing speed for the SDM will cause lower noise suppression performance and due to the operation of the repeater the SDM modulation noise will fold back and increase the in-band noise level. Thus, the SDM actually does not contribute to the suppression of harmonics.
Another drawback with SDM is that the modulation harmonics are too high and that they are not possible to be attenuated to requested level even with an external filter. In some application scenarios, not only in-band noise level but also out-of-band noise level should be as low as possible. Thus, these high modulation harmonics, which are located at modulation frequency spaced locations, create problems in the described arrangements according to conventional technology.
Modulation harmonics are quite common for DTX/DPAs which use a different modulation processing frequency different from the carrier frequency. Thus, once the modulation frequency is too small, modulation harmonics will be quite close to the band-of-interest and then the system bandpass filter will be quite hard to attenuate.